Haemophilus ducreyi is the etiologic agent of the genital ulcer disease chancroid. H. ducreyi makes a lipooligosaccharide (LOS) that chemically and immunologically resembles the human glycosphingolipid antigens, I and i. Like the mature I and i antigen, sialic acid is condensed on the terminal N-acetyllactosamine of the major glycoform of the LOS. By resemblish the human host, LOS may help the organism evade host defenses. We recently evaulated an H. ducreyi LOS mutant in the human challenge model of infection (GCRC 776). The mutant (3000HP-RSM2), in which D-glycero-D-manno-heptosyltransferase (losB) is insertionally inactivated, produces a truncated LOS that lack ther terminal N-acetyllactosamine, and should not be sialylated. However, inoculation of 35000HP-RSM2 caused a pustule formation rate similar to that caused by the parent. Some H. ducreyi strains make an alternative LOS that terminated in Gal, which theoretically could be sialylated, then 35000HP-RSM2 may be able to utilize human receptors for sialic acid. After we completed protocol 766, we identified the gene responsible for sialyltransferase activity, designed first, and constructed an isogenic H. ducreyi mytant in the sialyltransferase gene. The mutant cannot condense sialyltransferase mutant is impaired in its ability to infect human skin when compared to its isogenic parent. To test this hypothesis, we will compare the ability of the parent and the mutant to cause experimental infection in human subjects. These studies will conclusively establish whether sialylation of LOS plays a role in the ability of H.ducreyi to cause disease.